The present invention relates to the field of electroabsorption optical modulators and, more particularly, to the utilization of a microstrip circuit to provide broadband impedance matching between the modulator and an external driving signal source.
In a conventional arrangement of an electroabsorption optical modulator, the modulator is positioned on an optical substrate, with an input (cw) optical signal applied along the input facet of the optical device and an output, modulated signal exiting from the output facet of the optical device, the input and output facets being defined as a pair of parallel endfaces. An electrical modulating signal is coupled to a surface area of the electroabsorption optical modulator, where the presence of this electrical signal will alter the characteristics of the input optical signal so as to produce a desired modulated waveform in the output optical signal.
In most cases, a microstrip transmission line element is used to couple the electrical signal between an external signal source and the electroabsorption optical modulator, due to the high frequency of the modulation signal. In order to allow for optimum signal transfer from the external signal source to the optical modulator, it is beneficial to provide impedance matching between these elements to improve the return loss of the modulator (improved efficiency in the system). Previous attempts at providing such matching have heretofore concentrated on a narrowband approach and, therefore, have been successful in lowering the return loss at a single frequency (or an extremely narrow range of frequencies). U.S. Pat. No. 6,101,295, issued to Naoyuki Mineo et al. on Aug. 8, 2000, discloses one such narrowband approach. In this case, a microstrip transmission line is formed to include a pair of open stubs along one side of the transmission line. By carefully choosing and controlling the dimensions of the pair of open stubs (in terms of both length and width), Mineo et al. is able to improve the return loss to be maintained in the range of xe2x88x929 dB to xe2x88x9230 db over a narrow frequency range of 57.6-61.6 GHz.
However, as the frequency of operation extends beyond these bounds in either direction, the return loss associated with the Mineo et al. arrangement soon becomes unacceptable. In addition, this narrowband technique is based on small signal analysis, which assumes that the operating point of the modulator is independent of the applied signal.
Most telecommunications applications have now developed to the point of requiring broadband operation, as a result of increasing the number of data channels supported along a transmission line. Under these conditions, a narrowband matching scheme, such as proposed by Mineo et al., will not provide significant improvement in the return loss of an electroabsorption optical modulator. Thus, a need remains in the prior art for a broadband impedance matching circuit acceptable for use in telecommunications applications of an electroabsorption optical modulator.
The need remaining in the prior art is addressed by the present invention, which relates to the utilization of a symmetric microstrip transmission line to provide broadband impedance matching between an electroabsorption optical modulator and an external driving signal source.
In accordance with the present invention, a broadband circuit for providing impedance matching between a broadband electrical signal source and an optical device (such as an electroabsorption optical modulator) comprises a microstrip transmission line coupled at a first end to the optical device and coupled at a second, opposite end to the broadband signal source, the transmission line defined as comprising a predetermined length L and width W for supporting propagation of an electrical input signal from the broadband signal source, and at least three pair of open stubs, joined to the microstrip transmission line, with each stub in a pair of stubs having a substantially identical length l and width w and disposed on opposite sides of said transmission line in a symmetrical arrangement.
The utilization of multiple open stubs, in a symmetric arrangement, provides for a xe2x80x9cflatteningxe2x80x9d of the return loss response characteristic of the electroabsorption modulator over a broad frequency range, resulting in a broadband impedance matching arrangement.
An aspect of the present invention, as discussed in detail below, is that the application of an actual broadband data signal, through the inventive impedance matching arrangement and onto the modulator, will have the effect of reducing the capacitance of the device and therefore improve its performance.
Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.